Formic Acid As a Hydrogen Storage Medium: Ruthenium-Catalyzed Generation of Hydrogen from Formic Acid in Emulsions

Formic acid is decomposed to H-2 and CO2 in the presence of RuCl3 and triphenylphosphines in an emulsion. In situ formed ruthenium carbonyls, such as [Ru(HCO2)(2)(CO)(2)(PPh3)(2)] (1), [Ru(CO)(3)(PPh3)(2)] (2), and [Ru-2(HCO2)(2)(CO)(4)(PPh3)(2)] (3), and a large cluster, involving a Ru-12 core, were identified and structurally characterized from the reaction mixtures. The catalytic activity of the mono and binuclear complexes was also investigated and it was found that [Ru-2(HCO2)(2)(CO)(4)(PPh3)(2)] (3) shows high stability even at elevated temperatures and pressures and its activity is 1 order of magnitude lower than those measured for the mononuclear complexes. It was also attempted to use [Ru-(HCO2)(2)(CO)(2)(PPh3)(2)] (1) as a catalyst for the hydrogenation of CO2 to formic acid under neutral conditions. Although the reduction of CO2 did not take place, the conversion of [Ru(HCO2)(2)(CO)(2)(PPh3)(2)] (1) to an unexpected carbonate, [Ru(CO3)(CO)(2)(PPh3)(2)]center dot H2O was observed.